Fig. 3
From: Ring-shaped bifocal lens used for fluorescent self-referenced holographic imaging
a Wavefronts of a single-point source modulated by a Hariharan-Sen interferometer. b Wavefronts of a single-point source modulated by the RBL. At the detector plane, the yellow crosses denote the places, where the OPD between the two beams is zero, thus we have a constructive interference. The red lines (or dot that is a zero length line) originating from a pixel of the detector show the optical paths between the pixel and the pair of wavefronts. The length difference between such red lines is the OPD at the pixel in question